Optical pulses used in telecommunications through optical fibers have a spectrum that covers a frequency band which is very narrow (a few GHz) compared with the optical carrier (200 THz). They are usually described by their time profile, in other words by the photodetected signal, and are approximately characterized by their width at mid-height. But, when calculating the deformation caused by their propagation in optical fibers (chromatic dispersion, Kerr effect), it is also important to know the variation in the optical frequency (in other words the optical phase distortion) during the pulse (chirp). This variation is characterized approximately by measuring the product p=.DELTA.t.times..DELTA.f, where .DELTA.t=width of the photodetected pulse at mid-height and .DELTA.f=width of the optical spectrum at mid-height. This product p is minimum when the chirp is zero (constant optical frequency along the pulse), but this minimum is variable depending on the profile of the pulse.
In summary, the signal output by an optical emitter is a narrow band signal that is described by its complex envelope v(t), or by its complex optical spectrum V(f).
The easiest way of measuring optical pulses (modulus of v(t)) is to use a photodiode followed by a sampling oscilloscope. This is true for pulses wider than 20 ps, since the pulse response of the photodiode-oscilloscope pair is not less than 15 ps. The technique used for finer pulses consists of measuring the product or self-correlation function. But this only gives an estimate of the pulse width, since an assumption has to be made about the shape of the pulses.
The measurement of the phase v(t) is much more difficult. Several different methods are given in the literature, usually based on the principle of measuring the spectrum resolved in time.
FIG. 1 diagrammatically shows the principle described in the article by R. A. Linke et al. entitled "Modulation Induced Transient Chirping in Single Frequency Lasers", which was published in the IEEE Journal of Quantum Electronics, vol. QE 21, p. 593-597, 1985. Optical pulses are generated by an optical pulse generator 2. The pulses are filtered by a tunable filter 4 and are detected by a wide band photoreceiver 6, which produces signals sent on a sampling oscilloscope 8. A variable delay 10 triggers the time base of the oscilloscope 8. This method is limited to relatively wide pulses (wider than 30 ps), this limitation being imposed by the photoreceiver response time.
References in FIGS. 2 and 3 that are identical to or correspond to references in FIG. 1 denote the corresponding elements.
FIG. 2 shows the principle of the device described in the article by J. L. A. Chilla et al., entitled "Direct determination of the amplitude and the phase of femtosecond light pulses", Optics Letters, vol. 16, No. 1, pp. 39-41, 1991.
The filter 4 is a diffraction grating made tunable by moving a mirror. An optical gate 12 is controlled by the pulse to be measured and samples the filtered pulse. This gate inter-correlates the filtered pulse with the pulse to be measured. It is performed conventionally by generating the second harmonic of the optical frequency of the pulse to be measured, in a non-linear element. Reference 14 denotes a low frequency photoreceiver.
FIG. 3, also very schematically, shows the principle of the device described in the article by D. J. Kane et al., entitled "Characterization of Arbitrary Femtosecond Pulses Using Frequency-Resolved Optical Grating", IEEE Journal of Quantum Electronics, vol. QE-29, pp. 571-579, 1993.
An optical gate 12 is controlled by the pulse to be measured and samples the same pulse. In this case, the tunable optical filter 4 filters a sample of the pulse to be measured. As in the previous article, the "optical gate" function is obtained by generating the second harmonic in a non-linear element, in the same way as is done conventionally in an optical self-correlator.
In the techniques described above, the optical pulse to be measured is sampled in the frequency domain by a tunable filter (grating, spectrometer, spectrum analyzer), and then in the time domain (photodiode+ sampling oscilloscope or self-correlation product or inter-correlation product) or vice versa. The filter must be narrow if high resolution is required in the frequency domain. But in this case the pulse is wider. This results in poor resolution in the time domain. Since the time and frequency are two inverse magnitudes, it is impossible to precisely measure the instantaneous frequency variation along the pulse using the spectrum resolved in time method as described in the articles mentioned above. Typically, the measurement precision is estimated at 1/.DELTA.t (.DELTA.t=pulse width at mid-height).
The other disadvantages of these methods are as follows:
the propagation time of the tunable filter must be independent of the frequency, with a precision of less than about 1 fs; this is very difficult to achieve and this is why calibration is necessary, although the calibration is also difficult to implement, PA1 the optical pulse measurement requires double sampling (one in the frequency domain, and one in the time domain), causing energy dispersal and making high energy pulses necessary. This is even more true when the method uses non-linear effects, as in the articles by Chilla et al. and Kane et al. mentioned above, PA1 in the articles by Chilla et al. and Kane et al. described above, a search is made for the complex envelope v(t) that gives the best fit to the measurements. The solution found is not necessarily unique. PA1 means of converting the two spikes f.sub.n and f.sub.n+1, into two spikes with the same frequency: f.sub.n +0.5F PA1 and means of producing a signal representative of the phase of an interference signal between the two spikes with frequency f.sub.n +0.5F.
Another method of measuring the complex envelope is described in the article by B. S. Prade et al. entitled "A Simple method for the determination of the intensity and phase of ultrashort optical pulse", Optics Communications, pp. 79-84, 1994. The principle of this method consists of comparing the intensity spectrum of the pulse to be measured with the spectrum obtained after transmission in an optical fiber. These two spectra are different due to non-linear effects in the fiber (Kerr effect). A search is then made for a complex envelope v(t) which, by calculation, corresponds to these two spectra. But the solution is firstly not unique, and secondly is very sensitive to the measurement precision. It is very unusual to find the correct result, so that this method is unusable.